MOSFETs (metal oxide semi conductor field effect transistors) are widely used as electrically controlled switches. N channel MOSFETs, or NFETs, are used more often than P channel MOSFETs, or PFETs, because they have inherently lower ON resistance. NFETs lower ON resistance is because of the higher mobility of electrons vs. holes. NFETs require a positive gate voltage to allow current to flow from drain to source (“turn the NFET on” or “close the switch”).
A common switch application is one that switches power to a load, or closes a connection between a positive voltage source and a load sinking current to a lower potential, typically a ground. This is commonly referred to as a “high side” switch. If an NFET is to be used as a high side switch, the gate must be charged above the positive voltage source of the NFET sufficiently to turn on the NFET. This can be a difficult problem to solve as the most positive voltage in a system is commonly the supply voltage. Therefore, a method to control the gate of the NFET that is capable of driving the gate to a voltage above the positive voltage source must be used. This method must also be able to pull the gate voltage low enough to turn the NFET completely off. This requires a gate control circuit, which has two output states, 0V out and Vbat+Vgs where Vgs is the gate to source ON voltage and Vbat is the positive voltage source. Vgs is typically between 5 and 10V, but can be as high as 20V or as low as 1.8V.
Another common requirement of a switching device is to turn on and off rapidly. This could be to react to a fault condition where the opening or closing the switch would avert further damage. This high speed requirement could also be to achieve proportional control by means of Pulse Width Modulation (PWM). PWM is commonly achieved by pulsing the switch on and off, with the on time controlled with respect to the off time. In many applications the load to be powered requires proportional drive, or an amount of voltage somewhere between source voltage and ground. This creates an average voltage to the load between the source voltage and zero, or proportional control. By changing the on time with respect to the off time, the average voltage can be controlled.
The two requirements of driving the gate to a voltage above the source voltage and driving the source voltage rapidly commonly lead to an expensive solution. Therefore, there is a need in the art for a fast, low cost, high side NFET gate driver circuit that will improve upon the state of the art.
Thus, it is a primary object of the present invention to provide a method to control the gate of an NFET that is capable of driving the gate to a voltage above the positive voltage source in the circuit.
Another object of the present invention is to provide a high side NFET gate driver that can be manufactured at a low cost.
Yet another object of the present invention is to provide a fast, high side NFET gate driver.
A further object of the present invention is to provide a circuit that is able to hold the gate voltage low enough to turn off the NFET completely.
Another object of the present invention is to use the high side switch created in association with and drives the coil of an electrohydraulic valve.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.